Power supplies for LCD and LED displays. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Liquid crystal display (LCD) indicators and displays based on light emitting diodes (LED) can be operated from conventional power supplies. However, this is not the best way to supply power. Below will be shown options for switching on using specialized microcircuits - voltage regulators, which are produced by MAXIM.

Using a digital potentiometer to adjust the LED backlight

The 5-digit programmable potentiometer DS 1050 is being produced as the main element of the pulse-width modulator (PWM). Change the pulse width from 0 to 100% in steps of 3, 125%. The potentiometer is controlled by a two-wire serial interface compatible with I²C, addressing up to eight DS 1050s on a two-wire bus. The circuit solution for controlling the brightness of the LED backlight of the liquid crystal display is shown in fig. one.

Fig. 1.

This circuit is not designed to control the LCD contrast voltage. The 20x4 character display used in this example, type DMC 20481 from Optrex, has a yellow-green LED backlight. The forward voltage drop across the LEDs is 4,1 volts and the maximum forward current is 260mA.

By changing the duty cycle of the pulse-width modulator, thereby changing the input power to the LEDs. When the impulse is 100% of the mode cycle time, we have the maximum power supply and, accordingly, the maximum brightness of the glow. Conversely, when the cycle momentum is 0%, the brightness of the glow is also zero.

The control of the PWM modulator is quite simple. The only requirement is that the LEDs do not flash. Our eyes cannot see blinking at frequencies of 30 Hz and above. The "slowest" DS1050 operates at 1 kHz. This is quite enough for visual observation and minimization of electromagnetic radiation. MOS transistor Q1 must be selected so that it can be directly driven by a 5V pulse-width modulator whose voltage varies from ground to V_cc. The default PWM duty cycle at power-up is 2. The PWM-driven transistor Q1 can switch the 260 mA required for LED backlighting. The gate threshold voltage of transistor Q1 is 2-4 volts. Diode D1 type 1N4001 is used to lower Vcc to 4,3 volts, which is less than the maximum forward voltage drop of the LEDs. The resistor instead of the specified diode is not used due to the high power dissipation. To reliably close the MOSFET, a resistor R3 is installed, which eliminates the "floating" gate mode of Q1.

Capacitor C1 is used as a power filter, should work well at high frequency and is installed as close as possible to the terminals of U1, with a minimum distance to the power source.

Digital potentiometer DS 1050 - 001 is set by hardware with address A=000. The program for the microcontroller type 8051 can be found in the appendix to "App. note 163" on the MAXIM website.

To control the contrast of liquid crystal displays (LCDs), instead of traditional mechanical potentiometers, it is proposed to use a digital potentiometer such as DS1668/1669 Dallastats or DS 1803. The DS1668/1669 devices were chosen because they provide both push-button and microcontroller control of the current collector contact. It is also important that these devices have an internal non-volatile memory that allows you to save the position of the current collector without power supply. On fig. Figure 2 shows a schematic for LCD contrast control using a DS 1669 digital potentiometer.

Rice. 2. .

Of course, a double digital potentiometer type DS 1803 can also be used here.

The liquid crystal module (LCM) is powered by 5 volts. The same voltage is supplied to the DS 1669, whose resistance is 10 kOhm. The current collector terminal is connected directly to the power input V_o LCM drivers.

The use of a digital potentiometer allows you to reduce the size of the device, significantly increase the durability and transfer control to the system microcontroller.

Well, now back to the control of the LEDs. With the increasing popularity of color liquid crystal displays in mobile phones, PDAs, digital cameras, etc., white LEDs are becoming popular light sources.

White light can be provided by either cold cathode fluorescent lamps (CCFLS) or white LEDs. Due to its size, complexity, and high cost, CCFLS has long been the only source of white. But now they are losing ground to white LEDs. They do not require high voltage (200 - 500 VAC) and a large transformer to produce this voltage. And although the forward voltage drop on a white LED (3 to 4V) is higher than on a red (1,8V) or green (2,2 - 2,4V), they still require fairly simple power supplies. The brightness of a white LED is controlled by changing the current flowing through it. Full brightness occurs at 20 mA. As the current flowing through the LED decreases, the brightness decreases. Digital cameras and mobile phones typically require 2 to 3 LEDs. There can be 2 ways to group LEDs: parallel and serial.

When the LEDs are connected in series, the current through each will be guaranteed to be the same. But such an inclusion requires a higher voltage than with parallel connection. When connected in parallel, the voltage is approximately equal to the forward voltage drop across a single LED instead of the voltage drop across the entire row of LEDs. However, the brightness of the diodes can be different due to the spread of the forward voltage drop across the LEDs, hence different currents, if they are not regulated. The battery voltage in most cases is not enough to light up the white LED, so a DC/DC converter must be used. In this case, the parallel connection of LEDs is desirable, since DC / DC converters are most effective with a small ratio of increased output voltage to input voltage.

Parallel connection of LEDs

There are three main ways to connect LEDs in parallel, as shown in fig. 3.

Fig. 3.

1. Independent current regulation through each diode.
2. The currents are regulated by ballast resistors from a voltage regulated source corresponding to the forward voltage drop across the LED.
3. From a source with adjustable current, a voltage is obtained equal to the voltage drop across the adjustable LED and resistor, and with the help of ballast resistors, the current through the remaining LEDs is regulated.

Let's take a closer look at these options.

A simple way to control the current flowing through the LEDs is to use a chip specially designed for this purpose. The switching circuit is shown in fig. 4. Shown here is a cheap MAX1916 chip that allows you to adjust the current through 3 white LEDs. The absolute accuracy of the current is 10%, and the currents flowing through the LEDs differ by no more than 0,3%. This is the most important characteristic, since the luminous flux from each LED must be the same. At full brightness, the current through the LED is 20 mA. In this case, 225 mV is enough, exceeding the voltage drop across the LEDs, for the microcircuit to maintain the set current value. Setting the current through the LEDs is done using the resistor R_set. The equation for calculating the current is as follows.

where:
I_ice - current flowing through the LED [mA]
230 - chip conversion factor
U_out - output voltage of the regulator
U_set = 1, 215 V
R_set -resistor installed between the regulator output and the SET MAX1916 input (kΩ).

Fig. 4.

The absolute current must also be controlled, but the brightness will change in general for the entire device (for example, a phone display). The change in brightness can be obtained by applying to the enable (EN) input of the chip with a pulse-width modulation signal. The maximum brightness will be at 100% pulse width, and at 0% - the LED does not shine.

This switching method is less accurate, since the individual currents through each LED are not regulated. How can one increase the absolute accuracy of the currents flowing and matching them through each diode?

The current through the LED is calculated by the formula:

I_ice = (V_out - V_d) / R

Due to production variations, even at the same currents, the direct voltage drop across the LED (V_d) may be different. You can write the ratio of two currents through 2 diodes

I1/I2 = R2/R1 [(V_out - V_d1)/(V_out - V_d2)]

Taking into account that the resistors have high accuracy (this is acceptable), we have:

I1/I2 = (V_out - V_d1)/(V_out - V_d2)

It follows that the ratio (difference) of the currents through the diodes is the smaller, the higher the output voltage of the power source. It must be borne in mind that the convergence of the values ​​\u5b\uXNUMXbof the currents through the LEDs is paid for by a higher power consumption. Therefore, we can recommend a voltage at the output of the regulator equal to XNUMX volts.

To obtain such a voltage, you can use simple converters such as MAX 1595 (U_O = 5V, I_O = 125 mA), or use MAX1759 variable output transmitters. Thus, by changing the output voltage of the regulator, it is possible to correct the currents in the LEDs to the desired level (for example, 20 mA). If it is not possible to correct the current by adjusting the voltage at the output of the power supply, then resistors and MOS transistors are placed in parallel with the ballast resistors R1a: R3a, as shown in Fig. 5. Turning the MOS transistors on and off with a logic level, you can connect or disconnect additional resistors R1v:.R3v, effectively changing the value of the ballast resistor.

Fig. 5.

Using a converter with adjustable output current. On fig. 3c shows the principle of using a variable output current converter. In this scenario, the current through one of the diodes (fig. 3c - D1) is converted into a voltage drop across the resistor R1 and it is this voltage that is maintained by the converter. The converter can be a key type, switched capacitors or a linear regulator.

The equation for the current through the LED is the same as above.

I_x = (V_out - V_dx) / R_x (1)

But in this case V_out not adjustable, but I1 is adjustable and its value is

I1 = V_oc /R1(2)

where: V_oc - feedback voltage taken from the resistor R1.

Because the current of only one diode is regulated, the different forward voltage drops across the LEDs can cause different currents to flow through them. In this case, you can use the following. We divide the resistor into 2 parts: R1 \u1d R1A + R1B and substitute it in equation (1), and replace the value of R2 in equation (1) with R2B. R3 and R1 do not require resistor splitting. Their values ​​must be equal to R1A + R1B. Now the output of the regulator will maintain a voltage determined by the voltage drop across the resistor R6B, as shown in fig. 1. If the setting from R1B is equal to the voltage of RXNUMX, then the error amplifier will remain in the same state, the output voltage of the regulator will increase, which will ensure the matching of the currents through each LED.

Fig. 6.

Sequencing LEDs

The main advantage of connecting LEDs in a series chain is that the same current flows through all the diodes and the brightness of the glow is the same. The disadvantage with this inclusion: a higher voltage is required, since the voltage drop on each LED is summed up. Even 3 white LEDs require 9 - 12 volts. Usually, key regulators are used for such inclusion, as the most effective converters for these purposes. Figure 7 shows the connection diagram of the MAX 1848 key regulator, designed to control three white LEDs connected in series. The device can be powered from 2,6 to 5,5 volts with an output voltage of up to 13 volts. The input range is designed for one Li-ion battery or 3 NiCD/NiMH batteries. The operating frequency of the regulator is 1,2 MHz, which allows the use of external components with minimal dimensions. The output is a PWM signal. The excess voltage is rectified and fed to the LEDs. The current through the LEDs, and thus the brightness, can be adjusted using either a DAC-sampled voltage or a filtered PWM signal applied to the CTRL input of the MAX 1848. The MAX 1848 is up to 87% efficient with LEDs.

Fig. 7

For large displays where many LEDs are required, the MAX 1698 key controller can be used (see Figure 8). The microcircuit can operate from an input voltage of only 0,8 Volts, and the output voltage is limited by the operating voltage of an external n-channel MOSFET. Low, up to 300 mV feedback voltage (FB pin) contributes to the maximum efficiency of the circuit, which reaches 90%. The brightness of the LED is adjusted using a potentiometer, in which the brush is connected to the ADJ pin of the microcircuit. The potentiometer can be used both analog and digital.

Fig. 8

Of course, the number of chips that are used to power and backlight liquid crystal and LED displays is not limited to the names presented in the article. If the reader wants to select the microcircuits necessary for his particular case, then there is nothing easier than to enter the maxim-ic.com website and get acquainted with the characteristics of the products there.

Used information materials of the company MAXIM.

Author: A. Shitikov; Publication: radioradar.net

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